In the art of electrophotography, an electrophotographic plate comprising a photoconductive insulating layer on a conductive layer is imaged by first uniformly electrostatically charging surface of the photoconductive insulating layer. The plate is then exposed to a pattern of activating electromagnetic radiation such as light, which selectively dissipates the charge in the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image in the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic toner particles on the surface of the photoconductive insulating layer. The resulting visible toner image can be transferred to a suitable receiving member such as paper. This imaging process may be repeated many times with reusable photoconductive insulating layers.
Electrophotographic imaging members are usually multilayered photoreceptors that comprise a substrate support, an electrically conductive layer, an optional hole blocking layer, an adhesive layer, a charge generating layer, and a charge transport layer in either a flexible belt form or a rigid drum configuration. Multilayered flexible photoreceptor belts may include an anti-curl layer on the back side of the substrate support, opposite to the side of the electrically active layers, to render the desired photoreceptor flatness. One type of multilayered photoreceptor comprises a layer of finely divided particles of a photoconductive inorganic compound dispersed in an electrically insulating organic resin binder. The charge generating layer is capable of photogenerating holes and injecting the photogenerated holes into the charge transport layer.
As more advanced, higher speed electrophotographic copiers, duplicators and printers were developed, the performance requirements for the xerographic components increased. Moreover, complex, highly sophisticated, duplicating and printing systems employed flexible photoreceptor belts, operating at very high speeds, have also placed stringent mechanical requirements and narrow operating limits as well on photoreceptors.
Ideally, a photoreceptor can be charged capacitively with no dark decay. However, typically during the charging step, charge depletion results in voltage potentials that are less than the ideal capacitive value. Charge depletion is the difference between the capacitive value and the actual potential on a photoreceptor.
The ground plane of many modern photoconductive imaging members must be highly flexible, adhere well to flexible supporting substrates, and exhibit predictable electrical characteristics within narrow operating limits to provide excellent toner images over many thousands of cycles.
One type of ground plane is aluminum. However, aluminum films are relatively soft and exhibit poor scratch resistance during photoreceptor fabrication processing. Oxidation of the aluminum ground plane occurs as electric current is passed across the junction between the metal and photoreceptor, leading to degradation of image quality.
The oxides of aluminum which naturally form on the aluminum metal employed as an electrical blocking layer prevent charge injection during charging of the photoconductive device. If the resistivity of this blocking layer becomes too great, a residual potential will build across the layer as the device is cycled. Since the thickness of the oxide layer on an aluminum ground plane is not stable, the electrical performance characteristics of a composite photoreceptor undergoes changes during electrophotographic cycling. The accelerated oxidation of the metal ground plane increases optical transmission, causes copy quality nonuniformity and can ultimately result in loss of electrical grounding capability.
After long-term use in an electrophotographic copying machine, multilayered photoreceptors utilizing the aluminum ground plane may be observed to exhibit a dramatic dark development potential change between cycles.
One type of printable defect is small unexposed areas on a photoreceptor that fail to retain an electrostatic charge. These defects become visible to the naked eye after development with toner material. On copies prepared by depositing black toner material on white paper, these defects may be white or black depending upon whether a positive or reversal image development process is employed. In positive image development, these defects appear as white spots in the solid image areas of the final xerographic print. In other words, the image areas on the photoreceptor corresponding to the white spot fails to attract toner particles in positive write-reading image development. In reversal image development, black spots appear in background areas of the final xerographic copy. The white spots and black spots always appear in the same location of the final electrophotographic copies during cycling of the photoreceptor. The white spots and black spots do not exhibit any single characteristic shape, are small in size, and are visible to the naked eye.
Corrosion limits photoreceptor electrical life and causes printable defects. Therefore, methods for reducing corrosion that do not negatively impact on retention of electrostatic charge or the mechanical integrity of the substrate are needed. This invention answers that need.